One function of a headrest is to prevent the head of a vehicle occupant from being thrown to the rear in relation to the vehicle in a rear end crash. Typically headrests are mounted in a vertically adjustable manner on the seat back. Moreover, there are also headrests which are designed as an extended, fixed part of the seat of the vehicle seat. In the case of the headrests presently employed in vehicles, irrespectively of whether it is a question of an adjustable or a non-adjustable design thereof, the cervical vertebrae of the seat occupant are subjected to substantial strains in a rear end crash because the distance of the occupant's head from the headrest may be comparatively large (up to 150 mm) despite the occupant's back engaging the seat back.
In order to avoid such strains, there has already been a proposal to accommodate a gas bag in the headrest. In an accident, a gas generator fills the gas bag within a matter of milliseconds under the control of a sensor. Since, however, there can be substantial differences in the distance between the occupant's head and the gas bag, a very short distance, for example, between the occupant's head and the headrest may mean that the occupant's head is even caused to bounce off the gas bag, this causing an undesired strain.
Furthermore, equipment has been suggested using energy stored by means of a spring or by means of a pyrotechnic device to cause a support means operated by a suitable triggering device to approach the occupant's head and, on contacting the head, to immediately remain in the respective position thereof. For the control suitable sensors are arranged on the surface of the headrest. Although highly promising results have been obtained in the course of tests such systems have not gone into mass-production due to substantial technical expenditure.